Nonaqueous electrolyte secondary battery

ABSTRACT

A nonaqueous electrolyte secondary battery according to the invention comprises a positive electrode containing a positive electrode active material including lithium containing composite oxide having a layer crystal structure represented by a general formula of Li x Mn a Co b M c O 2 (0.9≦X≦1.1, 0.45≦a≦0.55, 0.45≦b≦0.55, 0&lt;c≦0.05 and 0.9&lt;a+b+c≦1.1 are set and M is at least one kind selected from Al, Mg, Sn, Ti and Zr), a negative electrode containing a negative electrode active material capable of intercalating and deintercalating lithium ion, a separator for separating the positive electrode from the negative electrode, and a nonacqueous electrolyte.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a nonaqueous electrolytesecondary battery comprising a positive electrode containing a positiveelectrode active material capable of intercalating and deintercalatinglithium ion, a negative electrode containing a negative electrode activematerial capable of intercalating and deintercalating lithium ion, aseparator between the positive electrode and the negative electrode, anda nonaqueous electrolyte.

[0003] 2.Description of the Related Art

[0004] For a battery to be used in portable electronic and communicatingequipment such as a small-sized video camera, a mobile telephone and anotebook personal computer, recently, a nonaqueous electrolyte secondarybattery represented by a lithium ion battery having an alloy or a carbonmaterial capable of intercalating and deintercalating lithium ion as anegative electrode active material and lithium containing compositeoxide, for example, lithium cobalt oxide (LiCoO₂), lithium nickel oxide(LiNiO₂) or lithium manganese oxide (LiMn₂O₄) as a positive electrodematerial has been put into practical use to be a battery having a smallsize, a light weight and a high capacity and capable of carrying out acharge and discharge.

[0005] Since lithium nickel oxide (LiNiO₂) in the lithium containingcomposite oxide to be used for the positive electrode material of thenonaqueous electrolyte secondary battery has a feature of a highcapacity and a drawback of a poor safety and a low discharge operatingvoltage, there is a problem in that it is inferior to the lithium cobaltoxide (LiCoO₂).

[0006] Moreover, lithium manganese oxide (LiMn₂O₄) has a rich source andis inexpensive and excellent in safety, and has a drawback that anenergy density is low and manganese itself is dissolved at a hightemperature. Therefore, there is a problem in that it is inferior to thelithium cobalt oxide (LiCoO₂). At the present time, accordingly, the useof the lithium cobalt oxide (LiCoO₂) to be the lithium containingcomposite oxide has been a mainstream.

[0007] Recently, a novel positive electrode active material such asolivine type LiMPO₄(M=Fe, Co) or 5V class LiNi_(0.5)Mn_(1.5)O₄ has beenstudied and attention has been paid to the same material to be apositive electrode active material for a next generation nonaqueouselectrolyte secondary battery. However, the positive electrode activematerial has a high discharge operating voltage of 4 to 5 V whichexceeds the withstand potential (decomposition potential) of an organicelectrolyte used in the nonacqueous electrolyte secondary battery. Forthis reason, a deterioration in a cycle is increased with a charge anddischarge. Therefore, it is necessary to optimize other batterycomponents, for example, the organic electrolyte so that there is aproblem in that a long time is taken to achieve practical use.

[0008] On the other hand, lithium—manganese composite oxide having a 3Vclass layer structure has been proposed. There is a problem in that thelithium—manganese composite oxide having the layer structure has a largedischarge capacity, while a discharge operating voltage tends to bedivided into two stages in a 4V region and a 3V region and a cycle isdeteriorated greatly. Moreover, the discharge is mainly carried out inthe 3V region.

[0009] Therefore, there is a problem in that it is hard to directlysubstitute the same composite oxide for the use of a nonaqueouselectrolyte secondary battery using, as a positive electrode activematerial, lithium cobalt oxide utilizing a 4V region which is currentlyput into practical use.

[0010] Under the circumstances, there has been proposed lithium—nickelmanganese composite oxide (LiNi_(0.5)Mn_(0.5)O₂) having a layerstructure. The lithium—nickel—manganese composite oxide(LiNi_(0.5)Mn_(0.5)O₂) having the layer structure includes a plateau ina 4V region and a discharge capacity per unit mass is comparativelyhigh, that is, 140 to 150 mAh/g, and thus has an excellentcharacteristic as a novel positive electrode active material and hasthereby been considered to be hopeful as one of the positive electrodeactive materials for a novel nonacqueous electrolyte secondary battery.

[0011] However, a positive electrode active material(LiNi_(0.5)Mn_(0.5)O₂) greatly takes over the characteristics of lithiumcontaining composite oxide mainly containing nickel in that an initialcharge/discharge efficiency is low, that is, 80 to 90%, a dischargeoperating voltage is slightly low as in lithium nickel oxide and a cyclecharacteristic is poorer than that of lithium cobalt oxide, and there isa problem in that it is necessary to improve the characteristics moregreatly.

[0012] On the other hand, JP-A-2001-23617 has proposed a lithiumsecondary battery in which a part of LiMnO₂ in lithium—manganesecomposite oxide (LiMnO₂) having a 3V class layer structure issubstituted for Al, Fe, Co, Ni, Mg or Cr to obtainLi_(X)Mn_(Y)M_(1−Y)O₂(O<X≦1.1, 0.5≦Y≦1.0) so that a high temperaturecharacteristic is improved. In the lithium secondary battery proposed inthe JP-A-2001-23617, there is a problem in that it is hard to directlysubstitute the same composite oxide for the use of the lithium secondarybattery utilizing, as a positive electrode active material, lithiumcobalt oxide using a 4V region because a discharge voltage ofLi_(X)Mn_(Y)M_(1−Y)O₂ to be used as a positive electrode active materialis low.

SUMMARY OF THE INVENTION

[0013] The invention has been made to solve the problem described aboveand has an object to provide a positive electrode active material havinga plateau potential in a 4V region which is almost equivalent to lithiumcobalt oxide and has a large discharge capacity to obtain a nonaqueouselectrolyte secondary battery which is excellent in a batterycharacteristic such as a cycle characteristic or a high temperaturecharacteristic.

[0014] In order to achieve the object, the invention provides anonaqueous electrolyte secondary battery comprising a positive electrodecontaining a positive electrode active material including lithiumcontaining composite oxide having a layer crystal structure representedby a general formula of Li_(x)Mn_(a)Co_(b)M_(c)O₂(0.9≦X≦1.1,0.45≦a≦0.55, 0.45≦b≦0.55, 0<c≦0.05 and 0.9<a+b+c≦1.1 are set and M is atleast one kind selected from Al, Mg, Sn, Ti and Zr), a negativeelectrode containing a negative electrode active material capable ofintercalating and deintercalating lithium ion, a separator forseparating the positive electrode from the negative electrode, and anonacqueous electrolyte.

[0015] When the a and b values of the positive electrode active materialrepresented by the general formula of Li_(X)Mn_(a)Co_(b)M_(c)O₂ rangefrom 0.45 to 0.55(0.45≦a≦0.55, 0.45≦b≦0.55), the layer crystal structureis also an α−NaFeO₂ type crystal structure (monoclinic structure), thepeaks of LiCoO₂ and Li₂MnO₃ are not observed and they have a singlephase so that a flat discharge curve is obtained. On the other hand,when the a and b values exceed the range of 0.45 to 0.55, the peaks ofthe LiCoO₂ and Li₂MnO₃ are generated so that a crystal structure has twophases or more so that the discharge curve also tends to be divided intotwo stages from the end of the discharge. As a result of an experiment,moreover, a discharge capacity, a discharge operating voltage and aninitial charge/discharge efficiency can be enhanced when the a and bvalues range from 0.45 to 0.55.

[0016] For this reason, it is necessary to carry out a synthesis suchthat the a and b values of the positive electrode active materialrepresented by the general formula of Li_(X)Mn_(a)Co_(b)M_(c)O₂ are setto 0.45≦a≦0.55 and 0.45≦b≦0.55, respectively. In this case, a compoundhaving such a layer crystal structure does not have many sites in whicha lithium ion can be inserted and desorbed as in spinel type lithiummanganese oxide. For this reason, the lithium ion is inserted anddesorbed into and from layers. Therefore, an x value of the positiveelectrode active material represented by Li_(X)Mn_(a)Co_(b)M_(c)O₂ is atmost 1.1. In a state in which the synthesis stage of the positiveelectrode active material, moreover, a lithium source has only thepositive electrode active material during the fabrication of a battery.In consideration thereof, therefore, it is required that the x valueshould be at least 0.9. In this respect, it is desirable that thesynthesis should be carried out to obtain the x value of 0.9≦x≦1.1.

[0017] It has been found that a heterologous element (M=Al, Mg, Sn, Ti,Zr) is added to a lithium—manganese—cobalt (Li—Mn—Co) composite oxide, apart of the composite oxide is substituted for the heterologous element(M=Al, Mg, Sn, Ti, Zr) to obtain Li_(x)Mn_(a)Co_(b)M_(c)O₂(M=Al, Mg, Sn,Ti, Zr) so that a capacity retention rate can be enhanced after hightemperature preservation. The reason is that a part of Li—Mn—Co basedcomposite oxide is substituted for the heterologous element (M) such asAl, Mg, Sn, Ti and Zr so that the crystallinity of the layer structureis stabilized.

[0018] In this case, when the composition ratio (substitution amount) ofthe heterologous element such as Al, Mg, Sn, Ti or Zr exceeds0.05(c=0.05), the crystal structure tends to have two phases or more. Ifthe amount of substitution of the heterologous element is too large, itis hard to maintain a crystal shape so that a capacity retention rateand an initial charge/discharge efficiency during the high temperaturepreservation are reduced. Consequently, it is necessary to set thecomposition ratio (substitution amount) of the heterologous element suchas Al, Mg, Sn, Ti or Zn to be 0.05 or less (0<c≦0.05). Although otherelements such as Ni, Ca and Fe were investigated as the heterologouselement, the effect of enhancing the capacity retention rate at time ofthe high temperature preservation could not be observed for the otherelements.

[0019] From this viewpoint, the positive electrode active materialrepresented by the general formula of Li_(x)Mn_(a)Co_(b)M_(c)O₂ can besynthesized to obtain 0.90≦x≦1.10, 0.45≦a≦0.55, 0.45≦b≦0.55 and 0<c≦0.05and the heterologous element (M) is to be selected from Al, Mg, Sn, Tiand Zr.

[0020] Furthermore, it was found that the layer crystal structure can bemaintained if the (a+b+c) value of the positive electrode activematerial represented by the general formula of Li_(x)Mn_(a)Co_(b)M_(c)O₂ranges from 0.90 to 1.10. On the other hand, if the (a+b+c) valueexceeds the range of 0.90 to 1.10, the peaks of LiCoO₂ and Li₂MnO₃appear in an X-ray diffraction peak to obtain a mixture with a crystalstructure having two phases or more. From this viewpoint, it isnecessary to prepare the (a+b+c) value of the positive electrode activematerial represented by the general formula of Li_(X)Mn_(a)Co_(b)M_(c)O₂to 0.90<a+b+c≦1.10. If the composition ratio of a and b ranges of0.9<a/b<1.1, the discharge capacity is enhanced. Therefore, it isdesirable that the synthesis should be carried out to obtain thecomposition ratio ranging within 0.9<a/b<1.1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a chart showing a discharge curve (the relationshipbetween a unit active material capacity (mAh/g) and a discharge voltage(an electric potential for a lithium counter electrode)), and

[0022]FIG. 2 is a chart showing the relationship between acharge/discharge cycle and a capacity retention rate.

[0023]FIG. 3 is a table showing a discharge capacity (mAh/g) for 1 g ofan active material of each of the positive electrodes in this invention.

[0024]FIG. 4 is a table showing a capacity retention rate after 500cycles in this invention.

[0025]FIG. 5 is a table showing a table of recovery discharge capacityand battery expansion rates of the preserved batteries in thisinvention.

[0026]FIG. 6 is a table showing a capacity retention rate after 500cycles in other embodiments in this invention.

[0027]FIG. 7 is a table showing relationship between values of thepositive electrode active material and the crystal shape in thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Subsequently, an embodiment of the invention will be describedbelow and the invention is not restricted to the embodiment but can beproperly carried out without changing the object of the invention.

[0029] 1. Preparation of Positive Electrode Active Material

(1) EXAMPLES 1 TO 5

[0030] After lithium hydroxide, manganese oxide and cobalt oxide weredissolved in caustic soda respectively, they were mixed to have apredetermined mole ratio based on a hydroxide conversion to obtain amixed solution. Subsequently, titanium oxide was added to and mixed withthe mixed solution to have 0.01 mole % for the mole ratio of cobalthydroxide to manganese hydroxide, and the mixture was then bakedprovisionally at a low temperature of approximately 500° C. Thereafter,the mixture was baked at a temperature of 800 to 1000° C. in theatmosphere so that a positive electrode active material(LiMn_(a)Co_(b)Ti_(0.01)C₂) according to each of examples 1 to 5 wasobtained. In the mixed solution, a positive electrode active material(LiMn_(0.45)Co_(0.5)Ti_(0.01)O₂) prepared to have the mole ratio of thelithium hydroxide, the manganese oxide and the cobalt oxide of 1:0.45(a=0.45):0.55 (b=0.55) based on the hydroxide conversion was set to be apositive electrode active material α1 according to the example 1.

[0031] Similarly, a positive electrode active material(LiMn_(0.475)Co_(0.525)Ti_(0.01)O₂) prepared to have a mole ratio of1:0.475 (a=0.475):0.525(b=0.525) was set to be a positive electrodeactive material α2 according to the example 2, a positive electrodeactive material (LiMn_(0.50)Co_(0.50)Ti_(0.01)O₂) prepared to have amole ratio of 1:0.50 (a=0.50):0.50(b=0.50) was set to be a positiveelectrode active material α3 according to the example 3, a positiveelectrode active material (LiMn_(0.55)Co_(0.45)Ti_(0.01)O₂) prepared tohave a mole ratio of 1:0.525(a=0.525):0.475(b=0.475) was set to be apositive electrode active material α4 according to the example 4, and apositive electrode active material (LiMn_(0.55)Co_(0.45)Ti_(0.01)O₂)prepared to have a mole ratio of 1:0.55(a=0.55):0.45(b=0.45) was set tobe a positive electrode active material α5 according to the example 5.

[0032] When the X-ray diffraction pattern of each of the positiveelectrode active materials α1 to α5 was obtained, the peaks of LiCoO₂and Li₂MnO₃ were not observed and an α-NaFeO₂ type crystal structure (alayer crystal structure having a single phase) was found.

(2) EXAMPLES 6 TO 10

[0033] Aluminum oxide was added to and mixed with the same mixedsolution as that in each of the examples 1 to 5 to have 0.01 mole % forthe mole ratio of cobalt hydroxide to manganese hydroxide, and themixture was then baked in the same manner as in each of the examples 1to 5 so that a positive electrode active material(LiMn_(a)Co_(b)Al_(0.01)O₂) according to each of examples 6 to 10 wasobtained. A positive electrode active material(LiMn_(0.45)Co_(0.55)Al_(0.01)O₂) prepared to have the mole ratio oflithium hydroxide, manganese oxide and cobalt oxide of1:0.45(a=0.45):0.55(b=0.55) based on the hydroxide conversion was set tobe a positive electrode active material β1 according to the example 6.

[0034] Similarly, a positive electrode active material(LiMn_(0.475)Co_(0.525)Al_(0.01)O₂) prepared to have a mole ratio of1:0.475(a=0.475):0.525(b=0.525) was set to be a positive electrodeactive material β2 according to the example 7, a positive electrodeactive material (LiMn_(0.50)Co_(0.50)Al_(0.01)O₂) prepared to have amole ratio of 1:0.50 (a=0.50):0.50(b=0.50) was set to be a positiveelectrode active material β3 according to the example 8, a positiveelectrode active material (LiMn_(0.525)Co_(0.475)Al_(0.01)O₂) preparedto have a mole ratio of 1:0.525(a=0.525):0.475(b=0.475) was set to be apositive electrode active material β4 according to the example 9, and apositive electrode active material (LiMn_(0.55)Co_(0.45)Al_(0.01)O₂)prepared to have a mole ratio of 1:0.55(a=0.55):0.45(b=0.45) was set tobe a positive electrode active material β5 according to the example 10.

[0035] When the X-ray diffraction pattern of each of the positiveelectrode active materials β1 to β5 was obtained, the peaks of LiCoO₂and Li₂MnO₃ were not observed and an α-NaFeO₂ type crystal structure (alayer crystal structure having a single phase) was found.

(3) EXAMPLES 11 TO 15

[0036] Magnesium oxide was added to and mixed with the same mixedsolution as that in each of the examples 1 to 5 to have 0.01 mole % forthe mole ratio of cobalt hydroxide to manganese hydroxide, and themixture was then baked in the same manner as in each of the examples 1to 5 so that a positive electrode active material(LiMn_(a)Co_(b)Mg_(0.01)O₂) according to examples 11 to 15 was obtained.A positive electrode active material (LiMn_(0.45)Co_(0.55)Mg_(0.01)O₂)prepared to have the mole ratio of lithium hydroxide, manganese oxideand cobalt oxide of 1:0.45 (a=0.45):0.55(b=0.55) based on the hydroxideconversion was set to be a positive electrode active material γ1according to the example 11.

[0037] Similarly, a positive electrode active material(LiMn_(0.475)Co_(0.25)Mg_(0.01)O₂) prepared to have a mole ratio of1:0.475(a=0.475):0.525(b=0.525) was set to be a positive electrodeactive material γ2 according to the example 12, a positive electrodeactive material (LiMn_(0.50)Co_(0.50)Mg_(0.01)O₂) prepared to have amole ratio of 1:0.50(a=0.50):0.50(b=0.50) was set to be a positiveelectrode active material γ3 according to the example 13, a positiveelectrode active material (LiMn_(0.525)Co_(0.475)Mg_(0.01)O₂) preparedto have a mole ratio of 1:0.525(a=0.525):0.475(b=0.475) was set to be apositive electrode active material γ4 according to the example 14, and apositive electrode active material (LiMn_(0.55)Co_(0.45)Mg_(0.01)O₂)prepared to have a mole ratio of 1:0.55(a=0.55):0.45(b=0.45) was set tobe a positive electrode active material γ5 according to the example 15.

[0038] When the X-ray diffraction pattern of each of the positiveelectrode active materials γ1 to γ5 was obtained, the peaks of LiCoO₂and Li₂MnO₃ were not observed and an α-NaFeO₂ type crystal structure (alayer crystal structure having a single phase) was found.

(4) COMPARATIVE EXAMPLES 1 TO 7

[0039] After lithium hydroxide, manganese oxide and cobalt oxide weredissolved in caustic soda respectively, they were prepared and mixed tohave a predetermined mole ratio based on a hydroxide conversion.Subsequently, the mixture was then baked provisionally at a lowtemperature of approximately 500° C. and was then baked at a temperatureof 800 to 1000° C. in the atmosphere so that a positive electrode activematerial (LiMn_(a)Co_(b)C₂) according to each of comparative examples 1to 7 was obtained. A positive electrode active material(LiMn_(0.40)Co_(0.60)O₂) prepared to have the mole ratio of lithiumhydroxide, manganese oxide and cobalt oxide of1:0.40(a=0.40):0.60(b=0.60) based on the hydroxide conversion was set tobe a positive electrode active material χ1 according to the comparativeexample Similarly, a positive electrode active material(LiMn_(0.45)Co_(0.55)O₂) prepared to be 1:0.45(a=0.45):0.55(b=0.55) wasset to be a positive electrode active material χ2 according to thecomparative example 2, a positive electrode active material(LiMn_(0.475)Co_(0.525)O₂) prepared to be 1:0.475(a=0.475):0.525(b=0.525) was set to be a positive electrode activematerial χ3 according to the comparative example 3, and a positiveelectrode active material (LiMn_(0.50)Co_(0.50)O₂) prepared to be1:0.50(a=0.50):0.50(b=0.50) was set to be a positive electrode activematerial χ4 according to the comparative example 4.

[0040] Furthermore, a positive electrode active material(LiMn_(0.525)Co_(0.475)O₂) prepared to be1:0.525(a=0.525):0.475(b=0.475) was set to be a positive electrodeactive material χ5 according to the comparative example 5, a positiveelectrode active material (LiMn_(0.55)Co_(0.45)O₂) prepared to be 1:0.55(a=0.55):0.45 (b=0.45) was set to be a positive electrode activematerial χ6 according to the comparative example 6, and a positiveelectrode active material (LiMn_(0.60)Co_(0.40)O₂) prepared to be1:0.60(a=0.60):0.40(b=0.40) was set to be a positive electrode activematerial χ7 according to the comparative example 7.

[0041] When the X-ray diffraction pattern of each of the positiveelectrode active materials χ1 to χ7 was obtained, the peaks of LiCoO₂and Li₂MnO₃ were not observed and a mixture with a crystal structurehaving three phases was found.

(5) COMPARATIVE EXAMPLES 8 AND 9

[0042] Titanium oxide was added to and mixed with the same mixedsolution as that in each of the examples 1 to 5 to have 0.01 mole % forthe mole ratio of cobalt hydroxide to manganese hydroxide, and themixture was then baked in the same manner as in the examples 1 to 5 sothat a positive electrode active material (LiMn_(a)Co_(b)Ti_(0.01)O₂)according to each of examples 8 and 9 was obtained. A positive electrodeactive material (LiMn_(0.40)Co_(0.60)Ti_(0.01)O₂) prepared to have themole ratio of lithium hydroxide, manganese oxide and cobalt oxide of1:0.40(a=0.40):0.60(b=0.60) based on the hydroxide conversion was set tobe a positive electrode active material χ8 according to the comparativeexample 8, and a positive electrode active material(LiMn_(0.60)Co_(0.40)Ti_(0.01)O₂) prepared to have a mole ratio of1:0.60(a=0.60):0.40(b=0.40) was set to be a positive electrode activematerial χ9 according to the comparative example 9. When the X-raydiffraction pattern of each of the positive electrode active materialsχ8 and χ9 was obtained, the peaks of LiCoO₂ and Li₂MnO₃ were observedand a mixture with a crystal structure having three phases was found.

(6) COMPARATIVE EXAMPLES 10 AND 11

[0043] Aluminum oxide was added to and mixed with the same mixedsolution as that in each of the examples 1 to 5 to have 0.01 mole % forthe mole ratio of cobalt hydroxide to manganese hydroxide, and themixture was then baked in the same manner as in the examples 1 to 5 sothat a positive electrode active material (LiMn_(a)Co_(b)Al_(0.01)O₂)according to each of comparative examples 10 and 11 was obtained. Apositive electrode active material (LiMn_(0.40)Co_(0.60)Al_(0.01)O₂)prepared to have the mole ratio of lithium hydroxide, manganese oxideand cobalt oxide of 1:0.40(a=0.40):0.60(b=0.60) based on the hydroxideconversion was set to be a positive electrode active material χ10according to the comparative example 10, and a positive electrode activematerial (LiMn_(0.60)Co_(0.40)Al_(0.01)O₂) prepared to have a mole ratioof 1:0.60(a=0.60):0.40(b=0.40) was set to be a positive electrode activematerial χ11 according to the comparative example 11. When the X-raydiffraction pattern of each of the positive electrode active materialsχ10 and χ11 was obtained, the peaks of LiCoO₂ and Li₂MnO₃ were observedand a mixture with a crystal structure having three phases was found.

(7) COMPARATIVE EXAMPLES 12 AND 13

[0044] Magnesium oxide was added to and mixed with the same mixedsolution as that in each of the examples 1 to 5 to have 0.01 mole % forthe mole ratio of cobalt hydroxide to manganese hydroxide, and themixture was then baked in the same manner as in the examples 1 to 5 sothat a positive electrode active material (LiMn_(a)Co_(b)Mg_(0.01)O₂)according to each of comparative examples 12 and 13 was obtained. Apositive electrode active material (LiMn_(0.40)Co_(0.60)Mg_(0.01)O₂)prepared to have the mole ratio of lithium hydroxide, manganese oxideand cobalt oxide of 1:0.40(a=0.40):0.60(b=0.60) based on the hydroxideconversion was set to be a positive electrode active material χ12according to the comparative example 12, and a positive electrode activematerial (LiMn_(0.60)Co_(0.40)Mg_(0.01)O₂) prepared to have a mole ratioof 1:0.60(a=0.60):0.40(b=0.40) was set to be a positive electrode activematerial χ13 according to the comparative example 13. When the X-raydiffraction pattern of each of the positive electrode active materialsχ12 and χ13 was obtained, the peaks of LiCoO₂ and Li₂MnO₃ were observedand a mixture with a crystal structure having three phases was found.

[0045] 2. Formation of Positive Electrode

[0046] By using the positive electrode active materials α1 to α5, β1 toβ5, γ1 to γ5 and χ1 to χ13 thus prepared as described aboverespectively, subsequently, a carbon conductive agent and a fluororesinbased binder were mixed with the positive electrode active materials α1to α5, β1 to β5, γ1 to γ5 and χ1 to χ13 in a constant rate (for example,a mass ratio of 92:5:3) to obtain a positive electrode mixture. Then,the positive electrode mixture was applied to both surfaces of apositive electrode formed of an aluminum foil and was thereafter dried,and subsequently, was rolled to have a predetermined thickness tofabricate positive electrodes a1 to a5, b1 to b5, c1 to c5 and x1 to x13respectively.

[0047] 3. Single Electrode Test

[0048] The positive electrodes a1 to a5, b1 to b5, c1 to c5 and x1 tox13 fabricated as described above were used and were accommodated in anopen type battery jar respectively by utilizing a lithium metal platefor their counter electrodes and reference electrodes, and anelectrolyte having LiPF₆ dissolved in a mixed solvent mixing ethylenecarbonate and diethyl carbonate in a volume ratio of 3:7 was injectedinto the battery jar so that an open type simple cell was fabricated.Subsequently, the simple cell thus fabricated was charged at a roomtemperature to 4.3 V for the counter electrode and was then dischargedto 2.85 V for the counter electrode to obtain a discharge capacity froma discharging time.

[0049] Moreover, a discharge voltage for the discharging time during thedischarge was measured to obtain a discharge curve and a dischargeoperating voltage. After a test, a discharge capacity (mAh/g) for 1 g ofan active material of each of the positive electrodes a1 to a5, b1 tob5, c1 to c5 and x1 to x13 was calculated so that a result shown in FIG.3 was obtained.

[0050] Furthermore, an initial charge/discharge efficiency was obtainedbased on the following equation (1) so that the result shown in FIG. 3was obtained.

Initial charge/discharge efficiency (%)−(discharge capacity/chargecapacity)X 100  (1)

[0051] In FIG. 3 showing FIG. 3, a, b, c and M indicate the a value, theb value, the c value and the hetelorogous element M, wherein thepositive electrode active material is represented by a general formulaof Li_(X)Mn_(a)Co_(b)M_(c)O₂.

[0052] The following was apparent from the result of FIG. 3. Morespecifically, when the a and b values of the positive electrode activematerial represented by the general formula of Li_(X)Mn_(a)Co_(b)M_(c)O₂range from 0.45 to 0.55, the discharge capacity, the discharge operatingvoltage and the initial charge/discharge efficiency are great, andfurthermore, an α-NaFeO₂ type crystal structure (monoclinic structure)is obtained as a layer crystal structure and the peaks of LiCo₂ andLi₂MnO₃ are not observed and a single phase is obtained. Consequently, aflat discharge curve was obtained.

[0053] On the other hand, when the a and b values exceed the range of0.45 to 0.55, the discharge capacity, the discharge operating voltageand the initial charge/discharge efficiency are reduced, andfurthermore, the peaks of LiCoO2 and Li₂MnO₃ are generated and acompound having a three-phase crystal structure is formed. Therefore, itcan be supposed that the discharge curve also tends to be divided intotwo stages at the end of the discharge and the crystal shape is changedto be orthorhombic. For this reason, it can be supposed that thedischarge capacity, the discharge operating voltage and the initialcharge/discharge efficiency are reduced.

[0054] Accordingly, it is necessary to carry out a synthesis such thatthe a and b values are set to 0.45≦a≦0.55 and 0.45≦b≦0.55, respectively.In this case, a compound having such a layer crystal structure does nothave many sites in which a lithium ion can be inserted and desorbed asin spinel type lithium manganese oxide so that the lithium ion isinserted and desorbed into and from layers. For this reason, the x valueof the positive electrode active material represented byLi_(X)Mn_(a)Co_(b)M_(c)O₂ is at most 1.1. In a state in the synthesisstage of the positive electrode active material, moreover, a lithiumsource has only the positive electrode active material during thefabrication of a battery. In consideration thereof, therefore, it isrequired that the x value should be at least 0.9. In this respect, it isdesirable that the synthesis should be carried out to obtain the x valueof 0.9≦x≦1.1.

[0055] Discharge curves (the relationship between a unit active materialcapacity (mAh/g) and a discharge voltage (an electric potential for alithium counter electrode) for a positive electrode using the positiveelectrode active material α3 (LiMn_(0.50)Co_(0.50)Ti_(0.01)O₂) accordingto the example 3, a positive electrode using lithium containingmanganese—nickel composite oxide (LiMn_(0.50)Ni_(0.50)O₂) to be atypical positive electrode active material, a positive electrode usingspinel type lithium manganese oxide (LiMn₂O₄) and a positive electrodeusing lithium cobalt oxide (LiCoO₂) were obtained as shown in a resultof FIG. 1. As is apparent from the result of FIG. 1, the positiveelectrode using the positive electrode active material α3(LiMn_(0.50)Co_(0.50)Ti_(00.01)O₂) according to the example 3 has a highdischarge operating voltage which is equivalent to that of each of thepositive electrode using the spinel type lithium manganese oxide(LiMn₂O₄) and the positive electrode using the lithium cobalt oxide(LiCoO₂) and has a plateau potential (a flat potential) in a 4V region.

[0056] On the other hand, it was found that the positive electrode usinglithium containing manganese—nickel composite oxide(LiMn_(0.50)Ni_(0.50)O₂) has a plateau potential in a 4V region, a lowdischarge operating voltage which is peculiar to an Ni system, and a lowinitial charge/discharge efficiency of approximately 85%. In contrast,the positive electrode using the positive electrode active material α3(LiMn_(0.50)Co_(0.50)Ti_(0.01)O₂) according to the example 3 had aninitial charge/discharge efficiency of 96.2% which is almost equal tothat of each of the positive electrode using the spinel type lithiummanganese oxide (LiMn₂O₄) and the positive electrode using the lithiumcobalt oxide (LiCoO₂). From this viewpoint, it is apparent that theLi—Mn—Co based positive electrode active material can have a greateradvantage than the Li—Mn—Ni based positive electrode active material inrespect of an electric potential and a capacity.

[0057] In general consideration of the results described above, it isnecessary to carry out the synthesis such that the x value of thepositive electrode active material represented by the general formula ofLi_(x)Mn_(a)Co_(b)M_(c)O₂ is set to be 0.9≦x≦1.1 and the a and b valuesare set to 0.45≦a≦0.55 and 0.45≦b≦0.55. Furthermore, the compositions ofthe positive electrode active materials α2 to α4, 2 to 4, 2 to 4 and χ3to χ5 having a very small reduction in a capacity are desirable and itis desired that the synthesis should be carried out to obtain thecomposition ratio of a to b ranging of 0.9<a/b<1.1.

[0058] 4. Investigation of Heterologous Element (M)

(1) POSITIVE ELECTRODE OF EXAMPLES 16 TO 20

[0059] After lithium hydroxide, manganese oxide and cobalt oxide weredissolved in caustic soda respectively, they were mixed to have a moleratio of the lithium hydroxide, the manganese oxide and the cobalt oxideof 1:0.49 (a=0.49):0.49(b=0.49) based on a hydroxide conversion toobtain a mixed solution. Subsequently, oxide containing a heterologouselement (M) was added to and mixed with the mixed solution to have 0.02mole % for the mole ratio of cobalt hydroxide to manganese hydroxide,and the mixture was then baked provisionally at a low temperature ofapproximately 500° C.

[0060] Thereafter, the mixture was baked at a temperature of 800 to1000° C. in the atmosphere so that a positive electrode active material(LiMn_(0.49)Co_(0.49)M_(0.02)O₂) δ1 to δ5 according to examples 16 to 20was obtained.

[0061] Subsequently, a carbon conductive agent and a fluororesin basedbinder were mixed with the positive electrode active materials δ1 to δ5in a constant rate (for example, a mass ratio of 92:5:3) to obtain apositive electrode mixture. Then, the positive electrode mixture wasapplied to both surfaces of a positive electrode formed of an aluminumfoil and was thereafter dried, and subsequently, was rolled to have apredetermined thickness to fabricate positive electrodes d1 to d5according to the examples 16 to 20. The positive electrode activematerial δ1 (LiMn_(0.49)Co_(0.49)Al_(0.02)O₂) according to the example16 uses aluminum (Al) as the hetelorogous element (M), the positiveelectrode active material δ2 (LiMn_(0.49)Co_(0.49)Mg_(0.02)O₂) accordingto the example 17 uses magnesium (Mg), the positive electrode activematerial δ3 (LiMn_(0.49)Co_(0.49)Sn_(0.02)O₂) according to the example18 uses tin (Sn), the positive electrode active material δ4(LiMn_(0.49)Co_(0.49)Ti_(0.02)O₂) according to the example 19 usestitanium (Ti), and the positive electrode active material δ5(LiMn_(0.49)Co_(0.49)Zr_(0.02)O₂) according to the example 20 useszirconium (Zr).

[0062] (2) Fabrication of Nonaqueous Electrolyte Secondary Battery

[0063] First of all, a negative electrode active material capable ofintercalating and deintercalating lithium ion and a styrene based binderwere mixed in a constant rate (for example, a mass ratio of 98:2) andwater was added to and mixed with them to obtain a negative electrodemixture, and the negative electrode mixture was then applied to bothsurfaces of a negative electrode formed of a copper foil and they wererolled to fabricate a negative electrode. For the negative electrodeactive material, a carbon based material capable of intercalating anddeintercalating lithium ion, for example, graphite, carbon black, coke,glassy carbon, carbon fiber or their baked product is suitable.Moreover, oxide capable of intercalating and deintercalating lithiumion, for example, tin oxide or titanium oxide may be used.

[0064] Subsequently, a lead was attached to each of the positiveelectrodes d1 to d5 fabricated as described above and the positiveelectrode x4 (using LiMn_(0.50)Co_(0.50)O₂ as a positive electrodeactive material) according to the comparative example 4 fabricated asdescribed above and a lead was attached to the negative electrodefabricated as described above, and the positive electrodes and thenegative electrode were spirally wound through a separator formed ofpolypropylene so that each spiral electrode member was obtained. Eachspiral electrode member was inserted into a battery armor can and eachlead was then connected to a positive electrode terminal or a negativeelectrode terminal. An electrolyte having LiPF6 dissolved in a mixedsolvent mixing ethylene carbonate and diethyl carbonate in a volumeratio of 3:7 was injected into the outer armor can and the outer armorcan was then sealed to fabricate nonaqueous electrolyte secondarybatteries D1 to D5 and X4 which have a capacity of 550 mAh,respectively. The battery can have any shape, for example, can be thin,square or cylindrical and a size thereof is not particularly restricted.

[0065] The nonaqueous electrolyte secondary batteries fabricated byusing the positive electrodes d1 to d5 were set to be batteries D1 to D5and the nonaqueous electrolyte secondary battery fabricated by using thepositive electrode x4 was set to be a battery X4. The electrolyte is notrestricted to the examples described above but LiClO₄, LiBF₄,LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂ or LiPF_(6−X)(C_(n)F_(2n+1))_(X)(1≦X≦6, n=1,2) is desirable for a Li salt (an electrolyte salt), for example, andone of them or more can be mixed for use. The concentration of theelectrolyte salt is not particularly restricted but 0.2 to 1.5 mol (0.2to 1.5 mol/l) per liter of an electrolyte is desirable.

[0066] Moreover, propylene carbonate, ethylene carbonate, butylenecarbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonateor γ-butyrolactone is desirable for a solvent, and one of them or morecan be mixed for use. A carbonate based solvent is preferred and it ispreferable that cyclic carbonate and acyclic carbonate should be mixedfor use. The propylene carbonate or the ethylene carbonate is preferablefor the cyclic carbonate, and the dimethyl carbonate, the diethylcarbonate or the ethyl methyl carbonate is preferable for the acycliccarbonate.

[0067] 5. Test

[0068] (1) Measurement of Capacity Retention rate

[0069] There was repeated a cycle test having, as one cycle, 4.2 V-500mA constant current—constant voltage charging and 500 mA constantcurrent discharging in which the batteries D1 to D5 and X4 fabricated asdescribed above were charged to 4.2 V with a charging current of 500 mA(1 It) in a room temperature (approximately 25° C.) atmosphere and wascharged at a 4.2 V constant voltage until a charging current of 25 mA orless was obtained after 4.2 V was reached, the charging was halted forten minutes, and they were then discharged with a discharging current of500 mA (1 It) to obtain a final discharging voltage of 2.75 V. Adischarge capacity after each cycle was obtained to calculate a capacityretention rate after each cycle (a capacity retention rate (%)=(adischarge capacity after each cycle/a discharge capacity after onecycle)X 100%). Consequently, a result shown in FIG. 2 was obtained.Moreover, when a capacity retention rate after 500 cycles wascalculated, a result shown in FIG. 4 was obtained.

[0070] As is apparent from the results shown in FIG. 2 and FIG. 4, aheterologous element (M=Al, Mg, Sn, Ti, Zr) was added to a Li—Mn—Cobased positive electrode active material and a part of them wassubstituted for the heterologous element (M=Al, Mg, Sn, Ti, Zr) toobtain a positive electrode active materialLiMn_(0.49)Co_(0.49)Al_(0.02)O₂, a positive electrode active materialLiMn_(0.49)Co_(0.49)Mg_(0.02)O₂, a positive electrode active materialLiMn_(0.49)Co_(0.49)Sn_(0.02)O₂, a positive electrode active materialLiMn_(0.49)Co_(0.49)Ti_(0.02)O₂, and a positive electrode activematerial LiMn_(0.49)Co_(0.49)Zr_(0.02)O₂. Thus, it is apparent that thecapacity retention rate can be enhanced. The reason is that a part ofthe Li—Mn—Co based positive electrode active material is substituted forthe heterologous element (M) such as Al, Mg, Sn, Ti or Zr to stabilizethe crystallinity of a layer structure.

[0071] While other elements such as Ni, Ca or Fe were also investigatedas the heterologous element, the effect of enhancing the capacityretention rate could not be observed. The reason is that a crystal shapeand a crystal size after the substitution have problems. In theserespects, it is necessary to carry out the synthesis such that the xvalue of the positive electrode active material represented by thegeneral formula of Li_(X)Mn_(a)Co_(b)M_(c)O₂ is set to 0.9≦x≦1.1 and thea and b values are set to 0.45≦a≦0.55 and 0.45≦b≦0.55 respectively andto select the heterologous element (M) from Al, Mg, Sn, Ti and Zr.

[0072] (2) High Temperature Preservation Characteristic after Charging

[0073] Moreover, the batteries D1 to D5 and X4 fabricated as describedabove were charged to 4.2 V with a charging current of 500 mA (1 It) ina room temperature atmosphere and were charged at a 4.2 V constantvoltage until a charging current of 25 mA or less was obtained after 4.2V was reached, and were then preserved for 20 days in a 60° C.atmosphere. An after-preservation discharge capacity was obtained from adischarging time when the batteries D1 to D5 and X4 after thepreservation were discharged to have a final discharging voltage of 2.75V with a discharging current of 500 mA (1 It), and a ratio to abefore-preservation discharge capacity was obtained to calculate acapacity retention rate (%). Thus, a result shown in FIG. 5 wasobtained. Moreover, they were discharged again to obtain a recoverydischarge capacity from the discharging time and a ratio to thebefore-preservation discharge capacity, thereby calculating a capacityrecovery rate (%). Thus, a result shown in FIG. 5 was obtained.Furthermore, when a battery expansion rate (a maximum value) wascalculated from an increase rate of the thickness of each of thebatteries D1 to D5 and X4 after the preservation (an increase rate ofthe thickness after the preservation for the thickness of each batterybefore the preservation). Thus, the result shown in FIG. 5 was obtained.

[0074] (3) High temperature Preservation Characteristic afterDischarging

[0075] Furthermore, the batteries D1 to D5 and X4 fabricated asdescribed above were charged to 4.2 V with a charging current of 500 mA(1 It) in a room temperature atmosphere and were charged at a 4.2 Vconstant voltage until a charging current of 25 mA or less was obtainedafter 4.2 V was reached and were then discharged with a dischargingcurrent of 500 mA (1 It) to obtain a battery voltage of 2.75 V, and werethereafter preserved for 20 days in a 60° C. atmosphere. The preservedbatteries D1 to D5 and X4 were charged and discharged again to obtain arecovery discharge capacity from a discharging time thereof and a ratioto a before-preservation discharge capacity, thereby calculating acapacity retention rate (%). Thus, a result shown in FIG. 5 wasobtained. Furthermore, when a battery expansion rate (a maximum value)was calculated from an increase rate of the thickness of each of thebatteries D1 to D5 and X4 after the preservation (an increase rate ofthe thickness after the preservation for the thickness of each batterybefore the preservation). Thus, the result shown in FIG. 5 was obtained.In the FIG. 5, a retention rate indicates the capacity retention rateand a recovery rate indicates the capacity recovery rate.

[0076] As is apparent from the result shown in the FIG. 5, at the 4.2 Vfinal charging after preservation, the batteries D1 to D5 have thecapacity retention rates and the capacity recovery rates improved moregreatly than those of the battery X4 and also have the expansion ratesmore reduced than the expansion rate of the battery X4 so that theeffect of suppressing gas generation can be enhanced. Also at the 2.75 Vfinal discharging after preservation, each of the batteries D1 to D5 hasthe capacity recovery rate improved more greatly than that of thebattery X4, and has the expansion rate more reduced than that of thebattery X4 so that the effect of suppressing the gas generation can beenhanced. At present, the detailed reason is not clear but it can besupposed that a coat is formed on the surface of a positive electrode tosuppress a reactivity with an electrolyte in addition to thestabilization of a crystal structure by the substitution of theheterologous element.

[0077] 6. Investigation of Amount of Substitution of HeterologousElement (M)

[0078] Subsequently, the amount of addition of a heterologous elementwas investigated.

(1) POSITIVE ELECTRODE ACTIVE MATERIAL ACCORDING TO EXAMPLES 21 TO 24AND COMPARATIVE EXAMPLE 14

[0079] After lithium hydroxide, manganese oxide and cobalt oxide weredissolved in caustic soda respectively, they were prepared and mixed tohave a predetermined mole ratio of the lithium hydroxide, the manganeseoxide and the cobalt oxide so that a mixed solution was obtained.Titanium oxide was added to and mixed with the mixed solution to have apredetermined mole ratio for cobalt hydroxide to manganese hydroxide,and the mixture was then baked provisionally at a low temperature ofapproximately 500° C. Thereafter, the mixture was baked at a temperatureof 800 to 1000° C. in the atmosphere so that a positive electrode activematerial according to each of examples 21 to 24 was obtained. A positiveelectrode active material (Li_(x)Mn_(a)Co_(b)Ti_(c)O₂) prepared to havex:a:b:c=0.495:0.495:0.01(a+b+c=1.00) was set to be a positive electrodeactive material (LiMn_(0.495)Co_(0.495)Ti_(0.01)O₂) ε1 according to theexample 21.

[0080] Similarly, a positive electrode active material prepared to havex:a:b:c=1:0.490:0.490:0.02(a+b+c=1.00) was set to be a positiveelectrode active material (LiMn_(0.490)Co_(0.490)Ti_(0.02)O₂) ε2according to the example 22, a positive electrode active materialprepared to have x:a:b:c=1:0.485:0.485:0.03(a+b+c=1.00) was set to be apositive electrode active material (LiMn_(0.485)Co_(0.485)Ti_(0.03)O₂)ε3 according to the example 23, and a positive electrode active materialprepared to have x:a:b:c=1:0.475:0.475:0.05(a+b+c=1.00) was set to be apositive electrode active material (LiMn_(0.475)Co_(0.475)Ti_(0.05)O₂)ε4 according to the example 24. Moreover, a positive electrode activematerial prepared to have x:a:b:c=1:0.450:0.450:0.10(a+b+c=1.00) was setto be a positive electrode active material(LiMn_(0.450)Co_(0.450)Ti_(0.10)O₂) χ14 according to the comparativeexample 14.

(2) POSITIVE ELECTRODE ACTIVE MATERIAL OF EXAMPLES 25 TO 28 ANDCOMPARATIVE EXAMPLE 15

[0081] Moreover, after lithium hydroxide, manganese oxide and cobaltoxide were dissolved in caustic soda respectively, they were preparedand mixed to have a predetermined mole ratio of the lithium hydroxide,the manganese oxide and the cobalt oxide so that a mixed solution wasobtained. Aluminum oxide was added to and mixed with the mixed solutionto have a predetermined mole ratio for cobalt hydroxide to manganesehydroxide, and the mixture was then baked provisionally at a lowtemperature of approximately 500° C. Thereafter, the mixture was bakedat a temperature of 800 to 1000° C. in the atmosphere so that a positiveelectrode active material according to each of examples 25 to 28 wasobtained. A positive electrode active material(Li_(x)Mn_(a)Co_(b)Al_(c)O₂) prepared to havex:a:b:c=1:0.495:0.495:0.01(a+b+c=1.00) was set to be a positiveelectrode active material (LiMn_(0.495)Co_(0.495)Al_(0.01)O₂) ζ1according to the example 25.

[0082] Similarly, a positive electrode active material prepared to havex:a:b:c=1:0.490:0.490:0.02(a+b+c=1.00) was set to be a positiveelectrode active material (LiMn_(0.490)Co_(0.490)Al_(0.02)O₂) ζ2according to the example 26, a positive electrode active materialprepared to have x:a:b:c=1:0.485:0.485:0.03 (a+b+c=1.00) was set to be apositive electrode active material (LiMn_(0.490)Co_(0.490)Al_(0.03)O₂)ζ3 according to the example 27, and a positive electrode active materialprepared to have x:a:b:c=1:0.475:0.475:0.05(a+b+c=1.00) was set to be apositive electrode active material (LiMn_(0.475)Co_(0.475)Al_(0.05)O₂)ζ4 according to the example 28. Moreover, a positive electrode activematerial prepared to have x:a:b:c=1:0.450:0.450:0.10(a+b+c=1.00) was setto be a positive electrode active material(LiMn_(0.450)Co_(0.450)Al_(0.10)O₂) χ15 according to the comparativeexample 15.

(3) POSITIVE ELECTRODE ACTIVE MATERIAL OF EXAMPLES 29 TO 32 ANDCOMPARATIVE EXAMPLE 16

[0083] Moreover, after lithium hydroxide, manganese oxide and cobaltoxide were dissolved in caustic soda respectively, they were preparedand mixed to have a predetermined mole ratio of the lithium hydroxide,the manganese oxide and the cobalt oxide so that a mixed solution wasobtained.

[0084] Magnesium oxide was added to and mixed with the mixed solution tohave a predetermined mole ratio for cobalt hydroxide to manganesehydroxide, and the mixture was then baked provisionally at a lowtemperature of approximately 500° C. Thereafter, the mixture was bakedat a temperature of 800 to 1000° C. in the atmosphere so that a positiveelectrode active material according to each of examples 29 to 32 wasobtained. A positive electrode active material(Li_(x)Mn_(a)Co_(b)Mg_(c)O₂) prepared to havex:a:b:c=1:0.495:0.495:0.01(a+b+c=1.00) was set to be a positiveelectrode active material (LiMn_(0.495)Co_(0.495)Mg_(0.01)O₂) η1according to the example 29.

[0085] Similarly, a positive electrode active material prepared to havex:a:b:c=1:0.490:0.490:0.02(a+b+c=1.00) was set to be a positiveelectrode active material (LiMn_(0.490)Co_(0.490)Mg_(0.02)O₂) η2according to the example 30, a positive electrode active materialprepared to have x:a:b:c=1:0.485:0.485:0.03(a+b+c=1.00) was set to be apositive electrode active material (LiMn_(0.490)Co_(0.490)Mg_(0.03)O₂)η3 according to the example 31, and a positive electrode active materialprepared to have x a:b:c=1:0.475:0.475:0.05(a+b+c=1.00) was set to be apositive electrode active material (LiMn_(0.475)Co_(0.475)Mg_(0.05)O₂)η4 according to the example 32.

[0086] Moreover, a positive electrode active material prepared to havex:a:b:c=1:0.450:0.450:0.10(a+b+c=1.00) was set to be a positiveelectrode active material (LiMn_(0.450)Co_(0.450)Mg_(0.01)O₂) χ16according to the comparative example 16.

[0087] When the X-ray diffraction patterns of the positive electrodeactive materials ε1 to ε4, ζ1 to ζ4 and η1 to η4 according to theexamples were obtained, the peaks of LiCoO₂ and Li₂MnO₃ were notobserved and an α-NaFeO₂ type crystal structure (a layer crystalstructure having a single phase) was found. Moreover, when the X-raydiffraction patterns of the positive electrode active materials x14 tox16 were obtained, the peaks of LiCoO₂ and Li₂MnO₃ were observed and amixture having a 3-phase crystal structure was found.

[0088] Subsequently, positive electrodes e1 to e4, f1 to f4 and g1 to g4were fabricated by using the positive electrode active materials ε1 toε4, ζ1 to ζ4, η1 to η4 and χ14 to χ16 in the same manner as describedabove, and nonaqueous electrolyte secondary batteries E1 to E4, F1 toF4, G1 to G4 and X14 to X16 were fabricated by using the negativeelectrode in the same manner as described above. The batteries E1 to E4,F1 to F4, G1 to G4 and X4 to X16 thus fabricated were charged to 4.2 Vwith a charging current of 500 mA (1 It) in a room temperature(approximately 25° C.) atmosphere and were charged at a 4.2 V constantvoltage until a charging current of 25 mA or less was obtained after 4.2V was reached, and the charging was then halted for 10 minutes, and theywere then discharged with a discharging current of 500 mA (1 It) until afinal discharging voltage of 2.75 V was obtained. Thereafter, an initialcharge/discharge efficiency was calculated based on the equation (1) sothat a result shown in FIG. 6 was obtained.

[0089] Moreover, there was repeated a cycle test having, as one cycle,4.2 V 500 mA constant current—constant voltage charging and 500 mAconstant current discharging in which the batteries E1 to E4, F1 to F4,G1 to G4 and X14 to X16 fabricated as described above were charged to4.2 V with a charging current of 500 mA (1 It) in a room temperature(approximately 25° C.) atmosphere and were charged at a 4.2 V constantvoltage until a charging current of 25 mA or less was obtained after4.2V was reached, the charging was halted for ten minutes, and they werethen discharged with a discharging current of 500 mA (1 It) to obtain afinal discharging voltage of 2.75 V. When a capacity retention rateafter 500 cycles (a discharge capacity after 500 cycles/a dischargecapacity after one cycle X 100%) was calculated, a result shown in FIG.6 was obtained. In FIG. 6, the positive electrode active materialaccording to the comparative example 4 is also indicated for the batteryX4 using x4.

[0090] As is apparent from the result shown in the FIG. 6, there arereduced the capacity retention rate and initial charge/dischargeefficiency of each of the batteries X14 to X16 using the positiveelectrode active materials x14 to x16 according to the comparativeexamples 14 to 16 in which the amount of substitution of theheterologous element such as Ti, Al or Mg is 0.10 mole % The reason isas follows. The crystal structure tends to have two phases or more whenthe amount of substitution of the heterologous element such as Ti, Al orMg exceeds 0.05 mole %. Therefore, when the amount of substitution ofthe heterologous element such as Ti, Al or Mg is too increased, it ishard to maintain the crystal shape. Therefore, it is necessary to setthe amount of substitution of the heterologous element such as Ti, Al orMg to be 0.05 mole % (c=0.05) or less.

[0091] 7. Relationship Between (a+b+c) Value and Crystal Shape

[0092] Subsequently, there have been investigated the (a+b+c) value ofthe positive electrode active material represented by the generalformula of Li_(x)Mn_(a)CobTic0 ₂ and the crystal shape.

[0093] First of all, lithium hydroxide, manganese oxide, cobalt oxideand titanium oxide were blended to obtain a composition (x=1.0, a/b=1,a≧0.45, b≦0.55, 0.0<c≦0.05) shown in FIG. 7 and were baked in the samemanner as described above so that positive electrode active materials θ1to θ5 according to examples 33 to 37 and a positive electrode activematerial χ17 according to a comparative example 17 were obtained.

[0094] Moreover, lithium hydroxide, manganese oxide, cobalt oxide andtitanium oxide were blended to obtain a composition (x=1.0, a≧0.45, b≦0.55, a>b, 0.0<c≦0.05) shown in FIG. 7 and were baked in the samemanner as described above so that positive electrode active materials τ1to τ5 according to examples 38 to 42 and a positive electrode activematerial χ18 according to a comparative example 18 were obtained.Furthermore, lithium hydroxide, manganese oxide, cobalt oxide andtitanium oxide were blended to obtain a composition (x=1.0, a≧0.45,b≦0.55, b<0.55, b>a, 0.0<c<0.05) shown in FIG. 7 and were baked in thesame manner as described above so that positive electrode activematerials κ1 to κ5 according to examples 43 to 47 and a positiveelectrode active material χ19 according to a comparative example 19 wereobtained.

[0095] As is apparent from the result of the FIG. 7, if the (a+b+c)value of the positive electrode active material represented by thegeneral formula of Li_(x)Mn_(a)Co_(b)Ti_(c)O₂ ranges from 0.90 to 1.10,a layer crystal structure can be maintained. On the other hand, if the(a+b+c) value exceeds the range of 0.90 to 1.10, the peaks of LiCoO₂ andLi₂MnO₃ appear in an X-ray diffraction peak so that a mixture with acrystal structure having two phases or more is obtained. Consequently,it is necessary to carry out preparation such that the (a+b+c) value ofthe positive electrode active material represented by the generalformula of Li_(x)Mn_(a)Co_(b)Ti_(c)O₂ is set to 0.90<a+b+c≦1.10.

[0096] As described above, in the invention, there is provided apositive electrode containing a positive electrode active materialcomprising lithium containing composite oxide having a layer crystalstructure represented by a general formula ofLi_(x)Mn_(a)Co_(b)M_(c)O₂(0.9≦X≦1.1, 0.45≦a≦0.55, 0.45≦b≦0.55, 0<c≦0.05and 0.9<a+b+c≦1.1 are set and M is at least one kind selected from Al,Mg, Sn, Ti and Zr). Therefore, it is possible to obtain a nonaqueouselectrolyte secondary battery which has a plateau potential in a 4Vregion that is almost equivalent to lithium cobalt oxide, has a largedischarge capacity and is excellent in a battery characteristic such asa cycle characteristic or a high temperature characteristic.

[0097] While the example in which the lithium hydroxide is used as thelithium source has been described in the embodiment, a lithium compoundsuch as lithium carbonate, lithium nitrate or lithium sulfate may beused in addition to the lithium hydroxide. Moreover, while the examplein which the manganese oxide is used as the manganese source has beendescribed, a manganese compound such as manganese hydroxide, manganesesulfate, manganese carbonate or manganese oxyhydroxide may be used inaddition to the manganese oxide. Furthermore, while the example in whichthe cobalt oxide is used as the cobalt source has been described, acobalt compound such as lithium carbonate, cobalt carbonate, cobalthydroxide or cobalt sulfate may be used in addition to the cobalt oxide.

[0098] Moreover, while the example in which the lithium hydroxide, themanganese oxide and the cobalt oxide are mixed in a state of hydroxide,the heterologous element is added thereto and is then baked has beendescribed in the embodiment, the lithium source, the manganese source,the cobalt source and the heterologous element may be baked in asolid-phase state.

[0099] Furthermore, while the example in which oxide such as Ti, Al, Mg,Su or Zr is added in the addition of the heterologous element such asTi, Al, Mg, Su or Zr has been described in the embodiment, the oxidesuch as Ti, Al, Mg, Su or Zr is not always required but sulfide such asTi, Al, Mg, Su or Zr or hydroxide such as Ti, Al, Mg, Su or Zr may beadded.

[0100] Moreover, while the example in which the invention is applied tothe nonaqueous electrolyte secondary battery using the organicelectrolyte has been described in the embodiment, it is apparent thatthe organic electrolyte is not restricted but the invention can also beapplied to a nonaqueous electrolyte secondary battery using a polymersolid electrolyte. In this case, it is preferable to use, as the polymersolid electrolyte, a gel-like solid electrolyte which is obtained bycombining a polycarbonate based solid polymer, a polyacrylonitrile basedsolid polymer, a copolymer comprising two kinds of them or more or acrosslinked polymer and a fluorine based solid polymer such aspolyvinylidene fluoride (PVdF), a lithium salt and an electrolyte.

What is claimed is:
 1. A nonaqueous electrolyte secondary batterycomprising a positive electrode containing a positive electrode activematerial capable of intercalating and deintercalating lithium ions, anegative electrode containing a negative electrode active materialcapable of intercalating and deintercalating lithium ions, a separatorbetween the positive electrode from and the negative electrode, and anonaqueous electrolyte, wherein the positive electrode active materialcomprises lithium containing composite oxide having a layer crystalstructure represented by a general formula of LixMnaCobMcO2(0.9≦X≦1.1,0.45≦a≦0.55, 0.45≦b≦0.55, 0<c≦0.05 and 0.9<a+b+c≦1.1 are set and M is atleast one kind selected from Al, Mg, Sn, Ti and Zr).
 2. A nonaqueouselectrolyte secondary battery comprising a positive electrode containinga positive electrode active material capable of intercalating anddeintercalating lithium ions, a negative electrode containing a negativeelectrode active material capable of intercalating and deintercalatinglithium ions, a separator between the positive electrode from and thenegative electrode, and a nonaqueous electrolyte, wherein the positiveelectrode active material comprises lithium containing composite oxidehaving a layer crystal structure represented by a general formula ofLi_(x)Mn_(a)Co_(b)M_(c)O₂(0.9≦X≦1.1, 0.45≦a≦0.55, 0.45≦b≦0.55, 0<c≦0.05,0.9<a+b+c≦1.1 and 0.9<a/b<1.1 are set and M is at least one kindselected from Al, Mg, Sn, Ti and Zr).